PS or PS based plastics are among the most commonly used thermoplastic polymers and generate a considerable amount of recoverable scrap material. Like other polymeric materials, such as polyethylene (PE), polypropylene (PP) and ethylene-vinyl acetate (EVA), due to their high stiffness over a range of service temperatures, PS would also seem very suitable for addition to bitumen and could thereby contribute effectively to the stiffness and strength of bitumen. Unfortunately, PS has problems similar to those of other polymeric materials, in that it does not significantly increase the elasticity of the asphalt binder and blends or dispersions with a range of different type of bitumens demix quite rapidly during hot storage. In addition, PS has been found to be very difficult to disperse into bitumen even using high shear, because of its intrinsic structural features of rigid molecular chains with high melt strength. Although it is economically attractive to consider PS based scrap thermoplastics as a modifier for asphalt, the above discussed obstacles have limited such an application.
The use of polymers (plastics or rubbers), whether singly or in combination, as asphalt modifiers has been known in the construction industry for many years. Many of these applications have received varying degrees of success in bituminous modification. The degree of success for each product is mainly dependent on the ease of processing, on the compatibility of the polymer with bitumen and on the final property of the bituminous composition. These prior art applications have provided the skilled technologist with many means for modifying bitumen properties, namely:
Furthermore, the prior art (U.S. Pat. Nos. 5,280,064 and 5,494,966, and assigned to the assignee hereof) also disclosed a reactive process of in-situ producing a copolymer highly miscible with bitumen from incompatible plastics (using PE of high MW) and compatibilized rubber components. The situ-prepared elastomer reagent was found to function as stabilizer for both PE based plastics and butadiene based elastomers when used alone or together as bituminous modifiers (WO 94/22957, assigned to the assignee hereof).
Although there are other types of polymers described in the prior art as modifiers for bituminous application, two types of polymers, namely PE and its copolymers (as a representative of the plastics field) and styrenic-diene copolymers (as a representative of the rubber field) still remain the most useful as modifiers in the asphalt industry, but only when they are stabilized or compatibilized in bitumen using technologies described elsewhere.
Polystyrene (PS) plastics represents a line of rigid polystyrene based products from crystal homopolymer, toughened graft copolymers, to blends or alloys with various rubbers. The rigidity and high melt strength of PS plastics causes them to be very difficult to disperse into hot liquid bitumen. The coarsely dispersed phase produced under high shear force separates quickly once agitation stops. Of all the prior art related to the use of plastics as bitumen modifiers, the inventor knows of none involving the use of any types of polystyrene rigid plastics for such purpose.
Of the prior art related to elastomeric modifiers, many have disclosed the application of styrenic block copolymers, commonly called thermoplastic rubbers (TR), which are produced by a sequential chemical operation of successive polymerisations of styrene-butadiene-styrene (SBS), styrene-ethylene/butylene-styrene (SEBS) and of styrene-isoprene-styrene (SIS) systems.
TR dispersions can render their strength and elasticity to bitumen from a physical crosslinking of styrene segments into a three-dimensional network as disclosed by Holden et al. in the Proceedings of International Rubber Conference, 1967 Maclaren. This result can be achieved by the agglomeration of the styrene polymerized block (or polystyrene segments of triblock copolymers), forming very fine domains (down to the nm scale), which provide the physical cross-linkage for a three-dimensional butadiene, ethylene/butylene or isoprene polymerized rubbery matrix. It is known, in all related prior art, that the chemical structure of the styrenic block copolymers which enhances bitumen performance modifier is not related to the chemical structure of the styrenic block copolymers used as a dispersing and/or compatibilizing agent which could promote dispersion and/or stabilization of another separate polymer. Specifically, most of the prior art is focused on processes by means of which the styrenic block copolymers may be compatibilized with bitumen.